Data communication between circuit boards installed in a shelf has traditionally been done over communications links located on the shelf backplane. Each board has an edge connected through an edge connector to the backplane, and data is transmitted to the backplane and received from the backplane through the edge connector. Improving technology has allowed packaging densities on circuit boards to continuously increase. This has resulted in the requirement for larger capacity communications links to the backplane and these usually take the form of additional pins on the edge connector. These additional pins result in the requirement for an increased insertion force to be applied to install the board, and this in turn requires the boards to be reinforced. As the number of edge connector contact pins increases, the possibility of contact failures also increases.
German patent DE3908786 to Cordell, July 1991, discloses the use of optical, or radio free-space interconnections between adjacent boards to relieve congestion on the backplane or to remove the necessity for edge connectors. Each board in the shelf has a transmitter and a receiver for transmitting and receiving signals to and from adjacent boards. In the case in which a particular board does not require any communications capability, a hole is made in the board such that a board on one side of that particular board can communicate with the board on the other side of that particular board. A major disadvantage of the Cordell system is that not all boards have the same transmit/receive components such that the position of the board is important which precludes interchangeability of the boards. Additionally, each board can only communicate with the nearest appropriately equipped boards. Furthermore, Cordell has no signal rerouting capability to cope with the failure of a board, and does not provide a simple method of testing boards.